Low density oriented strand board

ABSTRACT

A low density oriented strand board has an average density of less than about 1.5 times the density of wood species used in the board and a homogenous vertical density profile. Low density OSB may be formed by preheating the wood finish prior to pressing, controlling the moisture content such that the core layer has a higher moisture content than the face layers, or a combination of pre-heating and moisture content control.

FIELD OF THE INVENTION

[0001] The present invention relates to low density oriented strandboard and novel methods for making low density oriented strand board.

BACKGROUND OF THE INVENTION

[0002] The oriented strand board (“OSB”) industry emerged in the late1970s and soon became a major competitor to the plywood industry. By theyear 2000, OSB had already captured more than half of the North Americanstructural panel market. Although many efforts have been made by the OSBindustry to improve their products' properties, several OSB properties,such as the strength-to-weight ratio, homogeneous density profile, anddimensional stability, still compare unfavourably with plywood.

[0003] OSBs are manufactured from wooden strands combined with athermosetting resin and consolidated together under heat and pressure ina hot press. Typically, an OSB panel comprises a middle core layer andtwo outer face layers. In order to develop adhesive bonds between thewooden furnish, it is necessary to produce adequate contact between woodand resin, and raise the temperature to cure the resin. Currently,common commercial pressing operations use a closing time (press platensramp to the final position) in the range of 25 to 90 seconds. With thesedurations, the temperature in the middle layer (core) of the OSB isstill below the point necessary to soften the wooden furnish and curethe resin in the core layer. Additional time is necessary for the heatto transfer into the core to soften the wooden furnish and cure theresin. Because of this temperature gradient, strands in the surface andbottom layers of the mat that contacted to the hot platens first will besofter than those in the middle layer. When the pressure is applied tothe mat, the outside layers compress more than the middle core layer. Asa result, commercial OSBs typically have an “M” shape vertical densityprofile through the vertical direction (higher in the surface andbottom, and lower in the core) as shown in the prior art FIGS. 1 and 2.

[0004]FIG. 1 is the vertical density profile of a commercial {fraction(23/32)}″ OSB made with Southern Pine wood furnish. Although the averagedensity of this panel is 44 lb/ft³, the density of the surface andbottom layers can be as high as 57 lb/ft³ but the core layer is only 37lb/ft³. FIG. 2 is another commercial {fraction (23/32)}″ OSB made withAspen which average panel density is 35 lb/ft³. The highest density inthe outside layer is 45 lb/ft³ and the core layer is only 29 lb/ft³.Both panels have shown the typical “M” shapes in their vertical densityprofiles—very high density in the surface and bottom but a much lowerdensity in the core. There is currently no known technique to overcomethis issue and produce OSBs having a flatter vertical density profile.At the present time, the only way to make the adequate contact for theconsolidation is to use a much higher average density to raise thedensity in the core layer. In addition, when these panels are exposed towater, they would have very high thickness swelling due to the highcompression ratio in the surface and bottom layers.

[0005] Because of the uneven heating and resin setting nature of aconvention of OSB process, the moisture content is regulated such thatthe moisture content of the outer face layers is significantly higherthan the moisture content of the core layer. This is done to promoteheat transfer into the core layer by heat conduction.

[0006] The average density of conventional OSB is between 35 to 45lb/ft³ depending on the wood species used. When OSB manufacturersattempt to make a lower density OSB below this range, the first problemthey will confront would be a very lower density core with a porousappearance, therefore, causing problems of low strength properties.

[0007] Therefore, there is a need in the art for a method of making lowdensity OSB having a relatively homogenous vertical density profilewhich mitigates the difficulties in the prior art.

SUMMARY OF THE INVENTION

[0008] The present invention is directed to methods of making lowdensity OSB and the resulting low density OSB panels. Accordingly, inone aspect of the invention, the invention comprises a method of forminga low density OSB product, comprising the steps of:

[0009] (a) combining wood strands with thermosetting resin;

[0010] (b) preheating the wood strands-resin mixture; and

[0011] (c) pressing and heating the preheated wood strands-resin mixtureuntil the resin sets to form OSB.

[0012] In another aspect of the invention, the invention comprises amethod of forming a low density OSB product including a core layer andtwo outer face layers, comprising the step of controlling the moisturecontent of the core layer to be about equal to or higher than themoisture content of the outer face layers prior to pressing. Preferably,the moisture content of the core layer is between about 10% to about 30%while the moisture content of the outer face layers is below about 10%.More preferably, the moisture content of the core layer is about 20%while the moisture content of the outer face layers is about 8%.

[0013] In another aspect of the invention, the invention comprises a lowdensity OSB having a homogenous vertical density profile.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention will now be described by way of an exemplaryembodiment with reference to the accompanying simplified, diagrammatic,not-to-scale drawings. In the drawings:

[0015]FIG. 1 is a vertical density profile of prior art commercial OSBmade with Southern Pine.

[0016]FIG. 2 is a vertical density profile of prior art commercial OSBmade with Aspen.

[0017]FIG. 3 is a picture of a cross section of low density OSB madewith the pre-heated wood furnish as described herein.

[0018]FIG. 4 is a vertical density profile of a low density OSB madewith long Aspen strands.

[0019]FIG. 5 is a vertical density profile of a low density OSB madewith short Aspen strands.

[0020]FIG. 6 is a vertical density profile of two low density OSBs ofdifferent thicknesses made with moisture content controlled wood furnishand without preheating.

[0021]FIG. 7 is an example of a pressing cycle used to produce the lowdensity OSB of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The present invention provides a method of making a low densityOSB product by preheating the wood strands prior to consolidation. Asused herein, “low density” refers to OSB having an average density ofless than about 1.5 times higher than the density of the wood used inthe OSB, and preferably less than about 1.4 times higher. With typicalwood species used in OSB production, “low density” may refer to OSBproducts having an average density of less than about 40 lb/ft³,preferably less than about 35 lb/ft³, and more preferably around 30lb/ft³. For example, the density of aspen log is typically in the rangeof 22 to 25 lb/ft³, therefore a preferred “low density” aspen OSBproduct may have an average density of less than about 35 lb/ft³(25×1.4). The density of shortleaf Southern Pine is about 32 lb/ft³,therefore a preferred “low density” shortleaf Southern Pine OSB productmay have an average density of less than about 45 lb/ft³ (25×1.4).

[0023] As used herein, “homogenous vertical density profile” refers to adensity profile similar to that shown in FIGS. 4, 5 and 6, wherein thelowest core density is at least about 75% of the highest surfacedensity. Preferably, the lowest core density is greater than about 80%of the highest surface density. More preferably, it is greater thanabout 85% and most preferably it is greater than about 90%.

[0024] A feature of the present invention is a pre-heating procedureused with a conventional OSB production line to raise the temperature ofOSB strands before they are consolidated into the final product. Therange of the pre-heating temperature may be from about 35° C. up to theonset temperature of the particular adhesive or resin used in theproduct. After the pre-heating procedure, the softened strands in thecore layer will be easier to densify. With this invention, OSBmanufacturers no longer need to use a higher average panel density toraise the core density. As the result, the relatively high density inthe surface and core can be reduced or even eliminated.

[0025] The preheating process can be applied by any heat source, such asby microwave, radio frequency (RF) or high frequency irradiation,infrared irradiation, hot air, or steam, to bring up the strands'temperature. Any method of heat transfer, such as conduction, convectionor radiation may be used. The preheating process can be applied in anylocation in the production line before the final consolidation, such asheating the strands during the blending process, heating the mat duringthe mat formation, heating the mat after the mat formation but beforethe consolidation, or heating the mat before the final stage of theconsolidation. Because the target of the pre-heating step are thestrands in the core layer, the heating area of the OSB strands or matcan be either the entire mat or only the core zone.

[0026] This invention adds the pre-heating process to current productionlines in OSB mills for making low density OSBs but still withperformances able to pass standard requirements. In addition to asignificant reduction in density without significant loss of strengthand integrity, the OSBs made in accordance with this invention may alsohave a homogeneous vertical density profile as compared to conventionalOSB. Because of its low compression ratio in the surface and bottomlayers, OSB products made with this invention also have an excellentdimensional stability and low thickness swelling.

[0027] In an alternative embodiment, the moisture content of the corelayer and face layers may be manipulated to produce OSB products with ahomogenous vertical density profile, either with or without thepreheating step described herein. Preferably, the moisture content ofthe core layer is maintained between about 10% to about 30% while themoisture content of the outer face layers is below about 10%. Morepreferably, the moisture content of the core layer is between about 18%to about 22%, while the moisture content of the outer face layers isabout 8%. The mat with the controlled moisture contents is then pressed.Lower ramp pressures than conventional OSB pressing cycles may be usedbecause the softer core. Because of the lower density, the mechanism ofthe heat transfer during the pressing cycle in the low density OSB ismainly dependent on heat convection rather than the heat conduction asin the conventional OSBs. In spite of the higher moisture content in thecore layer than in the surface layers, the rate of the heat transferinto the core layer during the pressing cycle is higher. As a result,the vertical density profile is more homogenous than with conventionalprior art OSB methods.

[0028] In preferred embodiments, a method of the present invention maycombine the preheating steps and the moisture content control stepsreferred to above. Heating by microwave or RF irradiation worksparticularly well with higher moisture content in the core layer as theincreased moisture causes greater heat production in the core layer uponirradiation.

EXAMPLES

[0029] The following examples are intended to illustrate the claimedinvention, without limiting the invention to the specific elementsdescribed in the examples.

Example 1

[0030] Aspen strands having a length within a range of 5-5.75″, a widthwithin a range of 0.5-1″, and a thickness within a range of0.015″-0.020″ were used as the raw material. Seven weight percent of MDI(diphenylmethane diisocyanate) was applied as the binder to thesestrands when they tumble with the blender by means of a spinning disc.After blending, the furnish was formed into a mat similar toconventional OSB 3-layer orientation. The mat was pre-heated bymicrowave irradiation for 28 seconds until the core temperature in themat was raised to 53° C. The pre-heated mat then was pressed with thehydraulic press at a temperature of 200° C. for a period of 6 minutes.The thickness of the panel was 0.72 inches and the average density wastargetted at 30 lb/ft³.

[0031]FIG. 4 is the view of the cross section of the final product.Although its density is only 30 lb/ft³ (the actual density measured bythe QMS density profiler, Model QDP-01X is 29.2 lb/ft³), the sample hasdisplayed a very smooth appearance without any porous areas. FIG. 3shows the vertical density profile of the product. Unlike the “M” shapecommonly seeing in conventional OSB products, the panel has a very flator homogenous density profile. Table 1 is the testing result of theproduct. With the density of 30 lb/ft³, the panel has still passed allstandard requirements of CSA0437.0-9. The panel has an excellentdimension stability which thickness swelling is only 4.3%. TABLE 1Testing results of the low density OSB Requirement Testing Units O-2Direction Results Modulus of rupture psi 4200 Para 4430 1800 Perp 3020Modulus of elasticity psi x 800 Para 919 1000 225 Perp 300 Internal bondpsi 50.0 56.8 Thickness swell 24 h soak % 10 4.3 Water Absorption % NoReq. 21

Example 2

[0032] Shorter aspen strands having a length within a range of 4-4.5″, awidth within a range of 0.5-1″, and a average thickness of 0.022″ wereused as the raw material. Eight weight percent of MDI (diphenylmethanediisocyanate) was applied as the binder to these strands as they tumblewith the blender by means of a spinning disc. After blending, furnishwas hand-formed into a mat similar to the conventional OSB 3-layerorientation. The mat was pre-heated by a microwave oven for 22 secondsuntil the core temperature in the mat was raised to 55° C. Thepre-heated mat then was pressed with the hydraulic press at atemperature of 200° C. for a period of 6 minutes. The target thicknesswas 0.72 inches and density was 30 lb/ft³. Although its density is only30 lb/ft³ (the actual density measured by the QMS density profiler,Model QDP-01X is 29.3 lb/ft³), the panel has passed all standardrequirements of CSA0437.0-9 as shown in Table 2. FIG. 4 is the verticaldensity profile of the product in which the “M” shape density profilehas been reduced to minimum. The product has a homogeneous densityprofile. TABLE 2 Testing results of the low density OSB (short strands)Requirement Testing Units O-2 Direction Results Modulus of rupture psi4200 Para 4540 1800 Perp 2840 Modulus of elasticity psi x 800 Para 8191000 225 Perp 355 Internal bond psi 50.0 74.7 Thickness swell 24 h soak% 10 5.7 Water Absorption % No Req. 22.9

Example 3

[0033] Aspen strands having a length within a range of 5-5.75″, a widthwithin a range of 0.5-1″, and a thickness within a range of0.015″-0.020″ were used as the raw material. Seven weight percent of MDI(diphenylmethane diisocyanate) was applied as the binder to thesestrands when they tumble with the blender by means of a spinning disc.After blending, furnish was formed into the mat similar to conventionalOSB 3-layer orientation except that the moisture content of the corefurnish was adjusted to 20% while the moisture content of the facefurnish was adjusted to 8%. The mat was not pre-heated. The mat then waspressed with the hydraulic press at a temperature of 200° C. for aperiod of 3 minutes. The thickness of the panel was 0.72 ({fraction(23/32)}″) inches and the average density was targeted at 33 lb/ft³. Asimilar panel was pressed under similar conditions to a thickness of{fraction (7/16)}″ and an average density of 35 lb/ft³. FIG. 7 shows thepressing cycle conditions for each panel.

[0034]FIG. 6 shows the vertical density profile of the products. Unlikethe “M” shape commonly seeing in conventional OSB products, the panelshave a relatively flat or homogenous density profile. Table 3 includesthe testing results of the product. With an average density of 33 lb/ft³and 35 lb/ft³ respectively, the panels have still passed all standardrequirements of CSA0437.0-9. The panels have excellent dimensionstability in which thickness swelling is only 6.7% and 4.8%respectively. TABLE 3 (0437-1-93) O-2 Dir'n Group 1 Units Requirement7/16″ 23/32″ Modulus of rupture psi 4200 Para 7620 4600 1800 Perp 43403420 Modulus of elasticity psi × 800 Para 1222 991 1000 225 Perp 396 401Internal bond psi 50.0 98.2 45.7 Thickness swell 24 h soak % 10.0 6.74.8 Water Absorption % No Req. 21.7 17.7

[0035] As will be apparent to those skilled in the art, variousmodifications, adaptations and variations of the foregoing specificdisclosure can be made without departing from the scope of the inventionclaimed herein.

What is claimed is:
 1. A method of forming a low density OSB product,comprising the steps of: (a) combining wood strands with thermosettingresin; (b) preheating the wood strands-resin mixture; (c) pressing andheating the preheated wood strands-resin mixture until the resin sets.2. The method of claim 1 wherein the mixture is preheated to betweenabout 35° C. to about the onset temperature of the resin.
 3. The methodof claim 2 wherein the mixture is formed into a mat prior to bepreheated.
 4. The method of claims 1, 2 or 3 wherein the mixture or matis preheated by microwave or RF irradiation.
 5. The method of claims 1,2 or 3 wherein the mixture or mat is preheated by application of heatedair or steam.
 6. The method of claims 1, 2 or 3 wherein the mixture ormat is preheated by infrared irradiation.
 7. The method of claim 1wherein the mixture is preheated while the strands are combined with theresin.
 8. A method of forming a low density OSB product including a corelayer and two outer face layers, comprising the step of controlling themoisture content of the core layer to about equal to or higher than themoisture content of the outer face layers prior to pressing.
 9. Themethod of claim 8 wherein the moisture content of the core layer isbetween about 10% to about 30% while the moisture content of the outerface layers is below about 10%.
 10. The method of claim 9 wherein themoisture content of the core layer is about 20% while the moisturecontent of the outer face layers is about 8%.
 11. A low density OSBhaving a homogenous vertical density profile.
 12. The low density OSB ofclaim 11 wherein the average density is less than about 40 lb/ft³. 13.The low density OSB of claim 12 wherein the average density is less thanabout 35 lb/ft³.
 14. The low density OSB of claim 13 wherein the averagedensity is about 30 lb/ft³.
 15. The low density OSB of claim 11 whereinthe lowest core density is greater than about 80% of the highest surfacedensity.
 16. The low density OSB of claim 15 wherein the lowest coredensity is greater than about 85% of the highest surface density. 17.The low density OSB of claim 17 wherein the lowest core density isgreater than about 90% of the highest surface density.
 18. The lowdensity OSB of claim 11 wherein the average density of the OSB is lessthan 1.5 times the density of the wood species used in the OSB.
 19. Thelow density OSB of claim 18 wherein the average density of the OSB isless than 1.4 times the density of the wood species used in the OSB. 20.A low density OSB product formed by one of the methods of claims 1 to10.